1. Field of the Invention
The present invention relates to a liquid crystal display device that has an alignment film with improved transparency and improved high contrast performance.
2. Description of the Related Art
A liquid crystal display device has been used in various fields due to its features such as high display quality, a thin thickness, a light weight, and low power consumption. For example, the liquid crystal display device has been used as a monitor for a portable device such as a cellular phone and a digital still camera, a monitor for a desktop PC, a monitor for printing or designing, a monitor for a medical device, an LCD television, and the like.
As the liquid crystal display device has been used in various fields, high image quality and high quality thereof have been demanded. Particularly, high luminance and low power consumption thereof with high transmissivity have been strongly demanded. Further, there is a strong demand in which the liquid crystal display device needs to be supplied to the market at a low price.
Generally, in the liquid crystal display device, an alignment direction of a liquid crystal molecule changes when applying an electric field to the liquid crystal molecule of a liquid crystal layer interposed between a pair of substrates, whereby optical characteristics of the liquid crystal layer change, so that an image is displayed on the liquid crystal display device. When there is no application of an electric field, the alignment direction of the liquid crystal molecule is determined by an alignment film obtained by performing a rubbing treatment on a surface of a polyimide thin film.
In the active driving type liquid crystal display device having a switching element such as a thin film transistor (TFT) for each pixel, an electrode is provided in each of a pair of substrates sandwiching a liquid crystal layer, a so-called vertical electric field is set so that the direction of the electric field applied to the liquid crystal layer is substantially perpendicular to a surface of the substrate, and an image is displayed on the liquid crystal display device by using an optical rotary power of a liquid crystal molecule forming the liquid crystal layer.
As the representative the vertical electric field system type liquid crystal display device, a twisted nematic (TN) type is known. In the TN type liquid crystal display device, a narrow viewing angle is one of a number of problems. Therefore, an IPS (In-Plane Switching) type or an FFS (Fringe-Field Switching) type has been introduced into the market as a display type of realizing a wide viewing angle.
Each of the IPS type and the FFS type is a so-called horizontal electric field type in which a pectinate electrode is formed at one of a pair of substrates, and a generated electric field is substantially parallel to the surface of the substrate. Here, a liquid crystal molecule forming the liquid crystal layer is rotated within a plane substantially parallel to the substrate, and an image is displayed by using birefringence of the liquid crystal layer. This type has benefits that a viewing angle is wider than that of the TN type due to the in-plane switching of the liquid crystal molecule and load capacity is lower than that of the TN type. Due to such benefits, the horizontal electric field type has been expected to be a new liquid crystal display device which may be used instead of the TN type, and has been rapidly developed in recent years.
The liquid crystal display element controls the alignment state of the liquid crystal molecule inside the liquid crystal layer by the presence of the electric field.
That is, upper and lower polarizers provided outside the liquid crystal layer are disposed to be completely perpendicular to each other, and a phase difference is generated by the alignment state of the liquid crystal molecule therebetween, thereby forming light and dark state.
The transmissivity of the liquid crystal display element is largely dependent on not only light absorbing or scattering of various optical thin films such as a substrate, a transparent electrode, a liquid crystal layer, and a polarizer, but also light reflecting at a boundary surface originating from a difference in refractive index between optical thin films. The maximum refractive index may be obtained inside the optical thin film of the liquid crystal display element by indium tin oxide (ITO) having a refractive index set to 2.1 and used in a transparent electrode or silicon nitride (SiNx) having a refractive index set from 1.8 to 1.9 used in an interlayer isolation film that electrically isolates a pixel electrode and a common electrode from each other. Examples of the other members include an organic optical thin film such as a liquid crystal, an alignment film, a polarizer, or a retardation film, or a glass substrate, and the refractive index thereof is from about 1.4 to about 1.6.
The reflection loss between media having different refractive indexes may be effectively reduced by inserting various reflections preventing layers therebetween. As a representative reflection preventing layer, a multi-layer film such as a high refractive index layer and a low refractive index layer, a micro-lens array, or the like is used. However, since at least an optical structure having a wavelength order needs to be provided, it is difficult to provide the reflection preventing layer at a gap of about 100 nm at most between the transparent electrode and the liquid crystal layer of the liquid crystal display device. In order to make the thin film thinner and to equally prevent the reflection throughout the visible range, a grated refractive index (Grated Refractive Index, GRIN) thin film is used between the optical thin films having different refractive indexes to smoothly change the refractive index.
For example, the GRIN thin film is used to prevent connection loss at an optical communication fiber connection portion formed of inorganic glass (refer to Technical document 1: “Opt. Commun. 2002 28th Euro. Conf. Opt. Commun. (ECOC2002) 3 (2002) 1-2”), and is used in an external light prevention film formed of SiO2 glass in which the concentration of holes is controlled (refer to Technical document 2: “Appl. Opt. 42 (2003) 4573-4579”).
Further, JP 2007-248607 A suggests a structure in which a GRIN thin film formed of a SiOx thin film having a controlled oxidation degree is used in a liquid crystal display element.